Americans spend 74 minutes of their day online, with 14 of those minutes on Facebook (these are, of course, averages). He assumes only half of that Facebook time is used while online at work. So during work hours, people spend seven minutes of their work day on Facebook.

He then looks at the number of "knowledge workers," assuming those are the people with access to computers and Internet, which is about 30 per cent of working Americans. That equals about 48 million Internet-connected workers.

Multiply the number of connected workers by the number of minutes spent on Facebook, and that equals 336 million minutes on Facebook during the work day.

Fitzpatrick then 'guesstimates' the average salary to be $50,000 and 2,000 the average number of work hours, which amounts to an average employee salary of $0.40/minute.

That means every day, when we combine the seven minutes spent by every worker who is supposedly using Facebook, it's costing $140 million, or $28 billion a year.

There are definitely flaws with Fitzpatrick's model. For starters, while he's likely correct in his initial assumption that half of that Facebook time is spent in the office, he doesn't have any data to back it up, throwing a wrench into the formula.

But Fitzpatrick calls it "just a fun exercise" and says it shouldn't be a sign for thousands of companies to start blocking Facebook.